1. Field of the Invention
The present invention relates generally to lighting technique, and specifically to a light source apparatus and a microscope apparatus which extract and emit a specific type of light from light emitted from a light source.
2. Description of the Related Art
There is an increasing demand for microscopes which allow for an observation of a microscopic specimen at high magnification and a high resolution. One of such microscopes in use is an ultraviolet microscope in which ultraviolet radiation is used as illumination light to achieve a high resolution. The ultraviolet microscope uses ultraviolet radiation of a wavelength region ranging from a near-ultraviolet (NUV) region to a deep-ultraviolet (DUV) region. Generally, a wavelength region for which good performance of optical elements such as an objective lens used in the ultraviolet microscope is secured is a narrow wavelength region (e.g. 248±10 nm) centered around a predetermined ultraviolet wavelength (e.g., 248 nm). Therefore, the ultraviolet microscope needs a light source apparatus which extracts light of a predetermined ultraviolet wavelength region from broadband light that is emitted from a light source as suitable for an objective lens as mentioned above so as to emit the extracted light as illumination light.
One generally known technique for extracting light of a predetermined wavelength region from broadband light is to use a wavelength-selective transmission filter or a wavelength-selective reflection filter. The wavelength-selective transmission filter is generally capable of transmitting and extracting light of a predetermined wavelength region. The optical transmissivity of the wavelength-selective transmission filter varies depending on use, specification, design value and the like of each filter, and is approximately 80 to 90%, for example. When it is necessary to increase the wavelength selectivity so as to extract only the light of a predetermined wavelength region as far as possible, several wavelength-selective transmission filters may be used together. However, if two wavelength-selective transmission filters of 80% transmissivity are used together, for example, the total transmissivity is 64%. Then, the intensity of obtained light is not sufficient for the light source of the ultraviolet microscope. Thus, the wavelength-selective transmission filter is disadvantageous in comparison with the wavelength-selective reflection filter mentioned later in that a base material of the filter itself absorbs and reduces the light. Therefore, when the wavelength-selective transmission filter is used, it is difficult to obtain a bright observation image, and for such a purpose, expensive equipment such as a highly sensitive camera is necessary.
On the other hand, the wavelength-selective reflection filter can reflect the light of a predetermined wavelength region by reflectance of 90% or higher depending on the design value of each filter. However, the wavelength-selective reflection filter also reflects light outside the predetermined wavelength region by approximately a few percents. Therefore, the wavelength selectivity thereof is not sufficient in terms of strict extraction of light of only a desired wavelength region. Conventionally, the light emitted from the light source is reflected by the wavelength-selective reflection filters plural times so that the wavelength selectivity is enhanced and the intensity loss at the wavelength-selective filter is suppressed, whereby the light of a desired wavelength region is extracted.
JP-T-8-512137 (published Japanese translation of a PCT application) discloses a configuration of a lighting apparatus which spatially separates light from a light source by splitting the light into two fluxes in different directions with the use of a beam splitter, and extracts light of a desired wavelength with the use of a filter or a wavelength-selective mirror on each light path. The filter or the wavelength-selective mirror which is arranged securely on each light path determines the wavelength of the generated two beams of light. Thus, the lighting apparatus can emit light of two different wavelengths by switching.
Further, JP-A-6-18406 (Japanese patent application laid-open) discloses a light source apparatus which is configured to split light from a light source into two light paths and to combine two beams of light again. In each of the two light paths, a wavelength-selective element which extracts light of a predetermined wavelength region different from a wavelength region of light extracted by a wavelength-selective element in another light path, and a shutter are arranged. Further, another shutter is arranged across two light paths so as to work on both light paths simultaneously. The light source apparatus can emit one of the light beams extracted along the two light paths alternately at a high switching speed by rotationally driving the shutter arranged across the two light paths. Further, the light source apparatus can emit the light extracted along one light path alone by opening only the shutter on the pertinent light path.
Following requirements need to be met when desired light is to be extracted from the light emitted from the light source and to be used for lighting.
With regard to optical devices such as microscopes and examination devices, it is desirable that the light output from the lighting device have sufficient intensity. When light beams of plural wavelength regions are to be extracted from broadband light emitted from one light source, each light beam must be emitted without decrease in intensity as far as possible.
With regard to the apparatus mentioned above, when light beams of plural wavelength regions are to be extracted to be used as illumination light, each light source image corresponding to the light of a particular wavelength region must be focused in a telecentric manner, so that good optical characteristic can be obtained.
Further, it is desirable that the devices have a function of sequentially emitting extracted light beams of various wavelength regions at a high switching speed, or a function of emitting optionally combined light beams. The wavelength region of extracted light may be changed appropriately, or the number of wavelength regions of extracted light may be increased or decreased in design, depending on the type of observation. Even when the number of wavelength regions of light extracted from the light emitted from the light source is increased, for example, the apparatus needs to have a simple and compact configuration and be provided at low cost.